1. Field of the Invention
The present invention generally relates to an apparatus for continuously refining molten crude lead and more particularly to a furnace for the removal of antimony from the molten crude lead which may or may not contain arsenic and/or tin.
2. Description of the Prior Art
Softening, as the process is termed in this field of endeavor, is usually carried out in a reverberatory furnace or in a kettle as a batch process. In its simplest form, air is blown into the molten metal to oxidize the impurities which, dependent upon the temperature, rise to the surface of the bath in the form of a powdery scum or as a molten slag. If substantial quantities of tin are present, a dry powdery scum is always produced at the normal working temperature below 1000.degree. C. until the tin content, which oxidizes preferentially to arsenic and antimony, is reduced to the order to 0.2%. Below this amount, tin is oxidized together with arsenic and antimony and provided that the temperature is maintained at a high enough level, the oxidation products form a molten slag which floats on the surfaces of the metal.
The addition of fluxes such as caustic soda enable the oxidation to be carried out at lower temperatures thus making the process suitable for operation in a steel pot or kettle rather than in a more expensive refractory-lined furnace.
However, at nearly all lead smelters in the world, softening, whether at a higher temperature in a furnace or at a lower temperature in a kettle, is carried out as a batch operation which makes this process expensive in terms of the equipment used and the time required in carrying out the process. In one notable exception, air is blown into a bath of lead held in a small reverberatory furnace to oxidize the arsenic and antimony. Lead containing approximately 0.2% arsenic and 0.8% antimony is added to the bath at one end and softened lead with 0.03% antimony overflows through a specially shaped siphon device at the other. Lead is softened at the rate of nearly 30 tons per hour with a very small labor requirement and little other expense except for supplying the air and repairing the furnace. More importantly, good hygiene can be maintained around a small continuous unit compared with the difficulties of large batch units. One disadvantage of this type of operation is the necessity of maintaining the antimony content of the bath at the same level as desired in the softened product. In the case of the above mentioned process, the antimony content is maintained at 0.03%. It is well known in the art that a form of equilibrium exists between the antimony content of the lead oxide-antimony oxide slag and the antimony content of the lead that it is in contact with. Maintaining the bath of lead at a low antimony content inevitably means that the slag produced has a low antimony content or put in other terms for a given amount of antimony oxidized, a larger weight of by-product, which will incur an expense in its re-treatment will be produced. The relationship between the antimony content of a bath of lead and the slag in contact with it is well known and was presented in a publication of Dr. T.R.A. Davey-Proc. A.I.M.E. Symposium Lead-Tin Zinc '80 Las Vegas, February 1980, p. 489.